Protein ladder sequencing is a new technique developed in our laboratory to sequence polypeptides including those with modified amino acids. One of the important applications of this technique is the study of protein phosphorylation. To understand the specificities of different protein kinases and to further explore the potentials of ladder sequencing, a model peptide, which contains multiple serine-phosphorylation sites, was synthesized chemically. This peptide is being used to study the specificities of several different protein kinases and the stochiometries of phosphorylation. In addition, variously phosphorylated (site and number) model peptides (same sequence as above peptide) will be synthesized to examine whether our mass spectrometric technique can be used to obtain detailed information concerning the stochiometry of phosphorylation. A paper describing these results is in preparation.